Hello, DipStick,
I think you 2 are talking about the same thing, just nomenclature difference, like AOA.
You are right the 2 props have identical AOA as viewed from the shaft shaft axis.
But, the different airflow vectors that the upward prop sees vs. downward prop sees are the result vectors of props AOA w/r/t shaft axis PLUS the forward velocity of the plane. So as viewed from the ground, the downward prop will have a higher resultalt AOA.
This effect will happen only during transient stage of a maneuver, when the thrustline point higher than plane's motion, with airspeed significant fraction of prop speed. For a 3D plane doing a harrier, I don't think the airspeed is not quite enough to have great enough P-effect.
This brings up another point. For planes w/ massive built-in down thrust, like some trainers, won't the P-factor pull the plane to the right during steady-state forward flight?
Another thing I am curious about is the P-factor of a 3-bladed prop. I was told that a 3-bladed prop has much greater P-effect during pull-ups. It was an experiment by Andrew Jesky on a F3A plane. This probably requires more than hand calculation to understand the effect due to a lot of non-linearity involved. All the IAC planes use 3-blade, I am sure for ground-clearance reason. I wonder if it's an issue for them.
ORIGINAL: DipStick
Bill,
I question this statement:
For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch.
By pointing the nose up doesn't the angle of attack of the up or down going blade stay the same? The plane that the prop is rotating in and the direction of the thrust and drag will change. (put a ruler and prop on a screw driver, spin them together in any direction (direction of the nose) ...does the angle between the prop and the ruler change? The ruler represents the plane of the spin) As you increase forward velocity it will change the pitch of the prop. The faster you go the bigger the effect. If the prop is "nose up" then as you increase speed you will also increase the thrust differance between the two sides mainly because of the change in the realitive wind speed....change in AOA has little to no effect.
Steve